In general, exhaust gas is produced when diesel engines, boilers, incinerators and the like are operated. Nitrogen oxide (NOx) is contained in exhaust gas. A selective catalytic reduction (SCR) apparatus is used to reduce nitrogen oxide.
The selective catalytic reduction apparatus serves to inject a reducing agent into exhaust gas to reduce nitrogen oxide contained in exhaust gas by reducing the nitrogen oxide to nitrogen and water. Urea is used as the reducing agent for reducing nitrogen oxide.
In the selective catalytic reduction apparatus, a system, which injects a reducing agent by using urea as the reducing agent, refers to a reducing agent injection system (urea dosing system).
Regarding the reducing agent injection system, a technology, in which exhaust gas is heated by using a reducing agent pyrolysis (decomposition) system, and a reducing agent is injected into the heated exhaust gas to further promote a reducing action, has been known.
A configuration of the reducing agent pyrolysis system includes a blower, a heating device, a decomposition chamber, an ammonia injection grid (AIG), and a urea supply module. The blower supplies fresh air into the decomposition chamber. The heating device serves to thermally decompose urea, and an electric heater, an oil burner, a plasma burner, a gas type burner, or the like is used as the heating device. The decomposition chamber provides a space in which urea is thermally decomposed. The ammonia injection grid supplies a reactor with ammonia (NH3) which is produced by thermally decomposing urea. The urea supply module delivers a reducing agent from a urea tank to the decomposition chamber, and the urea supply module adopts a quantitative control pump or a mass flow controller (MFC).
An operation of the reducing agent pyrolysis system known in the related art will be described in accordance with an operational sequence.
A method of injecting ammonia (NH3) by using the reducing agent pyrolysis system is applied to a case in which a temperature of exhaust gas is approximately below 250° C. It has been known that a temperature of exhaust gas discharged from engines, boilers, incinerators and the like is below 250° C.
Fresh air is supplied into the heating device at a front end or a rear end of the reactor by using a blower.
The heating device heats the reducing agent to a temperature level from 400° C. to 600° C., and the heated reducing agent is supplied into the decomposition chamber.
The heated reducing agent is injected into the decomposition chamber by the urea supply module and the nozzle.
When the heated reducing agent is injected, the reducing agent is thermally decomposed into ammonia (NH3) and isocyanic acid (HNCO) by heat in the decomposition chamber. The ammonia and the isocyanic acid, which has been decomposed, are supplied into the reactor by the ammonia injection grid installed in the exhaust duct.
When the ammonia injection grid is applied, a distance from the reactor is shorter than that in a method of directly injecting the reducing agent.
However, in comparison with the method of directly injecting the reducing agent, the method of injecting ammonia by using the reducing agent pyrolysis system has problems in that a large number of additional components are required, the aforementioned many constituent elements thus need to be disposed, and as a result, a large space is occupied.
In addition, the reducing agent pyrolysis system known in the related art has problems in that piping is complicatedly configured to deliver the reducing agent between the constituent elements, and as a result, facility costs are increased.
In addition, the reducing agent pyrolysis system known in the related art has problems in that energy (electric power, fuel, or air pressure) and costs are additionally required to operate the blower and the heating device.
On the other hand, Korean Patent No. 10-1236305, Apparatus for Removing Nitrogen Oxides and Method for Removing Nitrogen Oxides Thereof, discloses a configuration in which an inner container is disposed in an outer container, and a plurality of holes is formed in the inner container. However, according to Korean Patent No. 10-1236305, a flow rate of exhaust gas flowing into the inner container cannot be controlled, and thus an appropriate mixture ratio cannot be provided, and as a result, there is concern that the reducing agent will leak into the outside in a case in which the amount of the reducing agent is excessively larger than the flow rate of exhaust gas.